The Na_<i>x</i>MoO₂ Phase Diagram (¹/₂ ≤ <i>x</i> < 1): An Electrochemical Devil’s Staircase

Layered sodium transition metal oxides represent a complex class of materials that exhibit a variety of properties, for example, superconductivity, and can feature in a range of applications, for example, batteries. Understanding the structure–function relationship is key to developing better materials. In this context, the phase diagram of the Na_<i>x</i>MoO₂ system has been studied using electrochemistry combined with in situ synchrotron X-ray diffraction experiments. The many steps observed in the electrochemical curve of Na_2/3MoO₂ during cycling in a sodium battery suggest numerous reversible structural transitions during sodium (de)­intercalation between Na_0.5MoO₂ and Na_∼1MoO₂. In situ X-ray diffraction confirmed the complexity of the phase diagram within this domain, 13 single phase domains with minute changes in sodium contents. Almost all display superstructure or modulation peaks in their X-ray diffraction patterns suggesting the existence of many Na_<i>x</i>MoO₂ specific phases that are believed to be characterized by sodium/vacancy ordering as well as Mo–Mo bonds and subsequent Mo–O distances patterning in the structures. Moreover, a room temperature triclinic distortion was evidenced in the composition range 0.58 ≤ <i>x</i> < 0.75, for the first time in a sodium layered oxide system. Monoclinic and triclinic subcell parameters were refined for every Na_<i>x</i>MoO₂ phase identified. Reversible [MoO₂] slab glidings occur during the sodium (de)­intercalation. This level of structural detail provides unprecedented insight on the phases present and their evolution, which may allow each phase to be isolated and examined in more detail

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions

Localization of Oxygen Interstitials in CeSrGa₃O_7+δ Melilite

The solubility of Ce in the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ and La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce³⁺ into Ce⁴⁺. Under CO reducing atmosphere, the La in LaSrGa₃O₇ may be completely substituted by Ce to form the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ solid solution, which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ compositions displayed much lower Ce solubility (<i>x</i> ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce³⁺ cations. At 600 °C, CeSrGa₃O_7+δ showed a conductivity of ∼10^–4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa₃O₇. The oxidation of Ce³⁺ into Ce⁴⁺ in CeSrGa₃O_7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa₃O_7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La_1.54Sr_0.46Ga₃O_7.27. Neutron powder diffraction study on CeSrGa₃O_7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO₄ polyhedral environment, leading to distorted GaO₅ trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa₃O_7+δ, in contrast with those in La_1+<i>x</i>Sr_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>}, may be correlated with the cationic size contraction from the oxidation of Ce³⁺ to Ce⁴⁺. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure

Crystal Structure and Luminescent Properties of Eu³⁺-Doped A‑La₂Si₂O₇ Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

Pure A-La₂Si₂O₇ powder has been synthesized through a spray pyrolysis method followed by calcination at 1100 °C for 15 h. The crystallographic structure, refined from the synchrotron powder diffraction pattern of the sample, showed tetragonal symmetry with space group <i>P</i>4₁, <i>a</i> = 6.83565(1) Å, and <i>c</i> = 24.84133(1) Å. The ²⁹Si and ¹³⁹La NMR spectra have been described here for the first time in the literature and could be simulated with four Si and four La resonances, respectively, in good agreement with the presence of four Si and four La crystallographic sites in the unit cell. The same synthesis method was successful for the synthesis of Eu³⁺-doped A-La₂Si₂O₇ (%Eu = 3– 40). The analysis of the unit cell volumes indicated that Eu³⁺ replaces La³⁺ in the unit cell for all Eu³⁺ substitution levels investigated. However, anomalous diffraction data indicated that the La/Eu substitution mechanism was not homogeneous, but Eu much prefers to occupy the RE3 sites. The Eu-doped A-La₂Si₂O₇ phosphors thus synthesized exhibited a strong orange-red luminescence after excitation at 393 nm. Lifetime measurements indicated that the optimum phosphor was that with an Eu³⁺ content of 20%, which showed a lifetime of 2.3 ms. The quantum yield of the latter was found to be 12% at 393 nm excitation. These experimental observations together with the high purity of the phase obtained by the proposed spray pyrolysis method make this material an excellent phosphor for optoelectronic applications

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

Crystal Structure and Luminescent Properties of Eu³⁺-Doped A‑La₂Si₂O₇ Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

Pure A-La₂Si₂O₇ powder has been synthesized through a spray pyrolysis method followed by calcination at 1100 °C for 15 h. The crystallographic structure, refined from the synchrotron powder diffraction pattern of the sample, showed tetragonal symmetry with space group <i>P</i>4₁, <i>a</i> = 6.83565(1) Å, and <i>c</i> = 24.84133(1) Å. The ²⁹Si and ¹³⁹La NMR spectra have been described here for the first time in the literature and could be simulated with four Si and four La resonances, respectively, in good agreement with the presence of four Si and four La crystallographic sites in the unit cell. The same synthesis method was successful for the synthesis of Eu³⁺-doped A-La₂Si₂O₇ (%Eu = 3– 40). The analysis of the unit cell volumes indicated that Eu³⁺ replaces La³⁺ in the unit cell for all Eu³⁺ substitution levels investigated. However, anomalous diffraction data indicated that the La/Eu substitution mechanism was not homogeneous, but Eu much prefers to occupy the RE3 sites. The Eu-doped A-La₂Si₂O₇ phosphors thus synthesized exhibited a strong orange-red luminescence after excitation at 393 nm. Lifetime measurements indicated that the optimum phosphor was that with an Eu³⁺ content of 20%, which showed a lifetime of 2.3 ms. The quantum yield of the latter was found to be 12% at 393 nm excitation. These experimental observations together with the high purity of the phase obtained by the proposed spray pyrolysis method make this material an excellent phosphor for optoelectronic applications

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

Crystal Structure and Luminescent Properties of Eu³⁺-Doped A‑La₂Si₂O₇ Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

Pure A-La₂Si₂O₇ powder has been synthesized through a spray pyrolysis method followed by calcination at 1100 °C for 15 h. The crystallographic structure, refined from the synchrotron powder diffraction pattern of the sample, showed tetragonal symmetry with space group <i>P</i>4₁, <i>a</i> = 6.83565(1) Å, and <i>c</i> = 24.84133(1) Å. The ²⁹Si and ¹³⁹La NMR spectra have been described here for the first time in the literature and could be simulated with four Si and four La resonances, respectively, in good agreement with the presence of four Si and four La crystallographic sites in the unit cell. The same synthesis method was successful for the synthesis of Eu³⁺-doped A-La₂Si₂O₇ (%Eu = 3– 40). The analysis of the unit cell volumes indicated that Eu³⁺ replaces La³⁺ in the unit cell for all Eu³⁺ substitution levels investigated. However, anomalous diffraction data indicated that the La/Eu substitution mechanism was not homogeneous, but Eu much prefers to occupy the RE3 sites. The Eu-doped A-La₂Si₂O₇ phosphors thus synthesized exhibited a strong orange-red luminescence after excitation at 393 nm. Lifetime measurements indicated that the optimum phosphor was that with an Eu³⁺ content of 20%, which showed a lifetime of 2.3 ms. The quantum yield of the latter was found to be 12% at 393 nm excitation. These experimental observations together with the high purity of the phase obtained by the proposed spray pyrolysis method make this material an excellent phosphor for optoelectronic applications